This patch supports the {get,set}_io_voltage operations of SDHI.

This operates the IOCTRL30 register on the R8A77990 SoC and makes
1.8V/3.3V signal voltage switch possible.
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>